Just to expand a little on dating during the first “floating J” series (1971 – 1972) 1J1 to 999J99 and why the position of the “J” is significant to where the s/n falls within the series. Here is what I believe the s/n sequence looks like:
1J1, 1J2, 1J3, 1J4, 1J5, 1J6, 1J7, 1J8, 1J9, 1J10, 1J11 . . . . . . . . . . 1J9999
2J1, 2J2, 2J3, 2J4, 2J5, 2J6, 2J7, 2J8, 2J9, 2J10, 2J11 . . . . . . . . . . 2J9999
3J1, 3J2, 3J3, 3J4, 3J5, 3J6, 3J7, 3J8, 3J9, 3J10, 3J11 . . . . . . . . . . 3J9999
. . .
10J1, 10J2, 10J3, 10J4, 10J5, 10J6, 10J7, 10J8, 10J9, 10J10 . . . . . 10J999
11J1, 11J2, 11J3, 11J4, 11J5, 11J6, 11J7, 11J8, 11J9, 11J10 . . . . . 11J999
. . .
99J1, 99J2, 99J3, 99J4, 99J5, 99J6, 99J7, 99J8, 99J9, 99J10 . . . . . . 99J999
100J1, 100J2, 100J3, 100J4, 100J5, 100J6, 100J7, 100J8, 100J9 . . . 100J99
101J1, 101J2, 101J3, 101J4, 101J5, 101J6, 101J7, 101J8, 101J9 . . . 101J99
. . .
999J1, 999J2, 999J3, 999J4, 999J5 . . . 999J99
By now you get the idea. Note in this first series of the floating J s/n format, there are a maximum of six characters . . . the letter J plus up to five numbers (there is a later floating J series with J plus six numbers)
I hope this helps and doesn't add to the confusion.
Russ
Edit to add: IF there were an equal number of “floating J-frames” produced in 1971 and 1972, the dividing line would be near the 56J prefix . . . meaning a 1J1 to 55J999 s/n would be 1971 and a 56J1 to 999J99 would be 1972.
If that’s not intuitive, let me know and I’ll share the spreadsheet where I did the math. Hint: 1J1 to 1J9999 has 10,000 combinations . . . and . . . 999J1 to 999J99 has 100 combinations.